In aluminum sheet and foil production processes, the sheet or foil is generally rolled into a coil and then annealed to provide desired mechanical properties. Typically, an annealing cycle involves heating the coils in a furnace to a given temperature and letting them “heat soak” for period of time. Both the steps of heating and soaking are performed in the furnace.
Operation and maintenance of such annealing furnaces is often very expensive and only a limited number are operated at a typical plant. The furnace is therefore often a limiting step in sheet and foil production and can cause bottlenecks. For example, a typical operation may use 4 batch type annealing furnaces, each furnace holding 3 coils. The heating step can take up to 8 hours, followed by a 4 hours heat soaking step. Therefore, of the total furnace cycle of 12 hours, one third is spent in non-furnace operations.
Attempts have been made in the past to avoid use of the furnace in the heat soaking step, so that the furnace can be used to heat the next batch of coils.
U.S. Pat. Nos. 5,655,593 and 5,894,879 disclose a rapid cooling process followed by casting that eliminates altogether heating and soaking steps, but do not discuss more efficient use of the furnaces.
U.S. Pat. No. 6,053,996 relates to the treatment of steel and refers to holding an intermediate slab in an enclosure for temperature homogenization, but does not teach methods of heat soaking for annealing purposes.
U.S. Pat. No. 6,579,387 and U.S. 2003/0173003 teach a continuous annealing process in which the strip is heated and then heat soaked in the same induction heater.
U.S. Pat. No. 4,846,675 discloses an enclosure used for annealing batches of metal coils. The enclosure is provided with a heating hood or a cooling hood, and would not be used exclusively for heat soaking.
British Patent 1 555 021 relates primarily to the treatment of steel and provides an enclosure that is used for both heating and heat soaking steps of annealing. U.S. Pat. Nos. 4,142,712 and 4,310,302 are further examples of coil enclosures used as heater for heat-treatment of metal coils.
It is therefore greatly desired to develop a method and apparatus to separate the heating step of a batch annealing process from the heat soaking step, thereby freeing the furnace for heating a next batch of coils.